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Notes of lectures & topic lectures Neural Basis of Cognition and Perception
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Summary Neural Basis of Cognition & PerceptionThe somatosensory system processes information that organisms “feel” & the
system is composed of diverse array of peripheral nerve endings specialized to detect
these sensors.
Human skin contains different kinds of sensory receptors that respond to various
mechanical, thermal or chemical stimuli. They then convey the info to the CNS (brain &
spinal cord) to areas where we perceive the stimuli.
The nerve endings of the sensory receptors transduce (convert) the stimuli into electrical
signals. The signals travel along axons (neuronal extensions) to the CNS.
A sensation transforms specific energy forms (stimuli) into neural signals, by
de/hyperpolarizing the membrane potential (transduction).
First the stimulus has to reach the threshold to fire an action potential (signal).
To perceive 2 different stimuli, we measure the just noticeable difference (JND).
Weber’s law shows the linear relationship between change in intensity required to produce
a JND, △I and the stimulus intensity, I → △I / I = k, in which k is a constant.
Fechner’s law shows the relationship between I and the neural signal, it is logarithmic. He
concluded that the difference in neural activity needed to distinguish between 2 different
stimuli is the same for all stimuli levels.
The absolute threshold is the smallest level at which a stimulus is detectable, but it is not
determined in a sharp way due to noise in the CNS.
Some responses are biased, for example an observer that responds to
the slightest indication, in that case you have more false positive
responses & a low threshold.
You can also have an observer that only responds when it is sure, in
that case you have more false negative responses & a high threshold.
There is a Gaussian distribution between the low thresholds & high
thresholds, with in the middle the ideal threshold.
The ideal observer minimizes the total errors & maximizes the correct
responses. Discriminability is the function of distance between peaks
& overlap (d’).
A stronger stimulus leads to a higher firing frequency, however the action potentials do not
get larger. The receptor potentials do get larger.
, The cell bodies of the neurons in a typical somatosensory
pathway are located in the dorsal root ganglion, the spinal
cord & the thalamus. The dorsal root ganglia (DRG) have
cell bodies outside of the spinal cord.
A stimulus opens the ion channels of encapsulated (myelin)
afferent fibers which results in an influx of Na+ ions into the
afferent fiber, resulting in a receptor potential.
There are 3 ways the ion channels can be activated. Direct
activation through lipid tension, which happens for
osmosis. Direct activation through structural proteins that
are linked to the ion channel. Indirect activation through
the membrane structural proteins, which needs “force”.
The receptive field of sensory neurons contains the sensory
receptors that feed into sensory neurons. It is the spatial
domain in the organ where stimulation excites/ inhibits the
neuron, so where the nerve endings are.
As mentioned before, we would like to know the stimulus needed to perceive 2 simultaneous
stimuli. These need a minimum interstimulus distance to distinguish them. There is an
inherent distance you can feel (the type of task doesn’t really matter). There is an
interrelationship between 2-point discrimination threshold, receptive field size (your
back is larger than your finger, the smaller the better discrimination), periphery receptor
density (the higher, the better you can distinguish both stimuli) & cortical magnification
factor (how many neurons in area of somatosensory cortex are responsible for processing
stimulus). Lateral inhibition happens when a neuron’s response to a stimulus is inhibited by
the excitation of a neighboring neuron. This increases the contrast & resolution of visual
stimuli. Some neurons adapt slowly (transient neurons), while others adapt rapidly, this is
called the temporal response.
Different sensations need different pathways. The detection of shape & size of objects is
done via tactile reception. The detection of tissue damage
(pain) is done via nociception. The detection of heat & cold
is done by thermoreception.
The mechanoreceptors encapsulate (myelinate) the
afferent fibers and turn them to the specific features of
somatic stimulation. These receptors are for tactile
sensation. The unmyelinated fibers have free nerve endings,
are slower & thinner so they conduct pain & temperature
(nociception & thermoception).
The mechanoreceptors use haptics, which is the active
touching for interpretation of spatiotemporal patterns of
stimuli. There are 4 types of mechanoreceptors. Merkel’s
disk, which provides info about the form. Meissner’s
corpuscles, which provides info about slippage. Pacinian
corpuscles, which provide info about vibrations. Ruffini’s
corpuscles, which provides info about stretch.
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